The P-V rating is commonly used to describe overall optical quality. Strehl Ratio – The peak intensity of a point-source image produced by the optic compared to that produced by a perfect system.Peak-To-Valley – P-V error is difference between the highest point and the lowest point on the surface.Root-Mean-Square – RMS error is a form of average error measure over the entire surface.There are also three measurements of overall quality of the optic: This measurement can be useful to check off-axis performance. Note: if you are testing an optical flat, do not subtract defocus you need to know if the flat has any power.Ĭoma is often caused by misalignment between the interferometer and the test optic. These aberrations are usually subtracted this is easily done in Quick Fringe. Since these aberrations are due to the adjustment of the interferometer relative to the test optic, neither of these two really have anything to do with the test optic. Defocus is the adjustment required to reach best focus for the optic. As long as the tilt is relatively small, it has no effect on the measurement. The tilt is introduced deliberately to make the measurements possible. The first two of these, tilt and defocus, are due to the setup of the interferometer. – A measure of the "potato chip" shape of the optic - how much more curvature it has in one direction than another. The surface is bent more in one direction than the other. Astigmatism An optical distortion where the image plane looks like a potato chip.This occurs in off-axis images on Newtonian telescopes. – An aberration which results in a pear-shaped image. This is commonly found in the off-axis images of fast Newtonian reflectors. Coma An optical aberration which results in a pear-shaped image.Spherical Aberration – A measure of the curvature of the wavefront.This aberration is usually subtracted from the interferogram, unless an optical flat is being tested. Defocus The most common optical aberration: lack of good focus.– How much the wavefront is tilted by the interferometer. In a telescope, this is removed by collimation. A small tilt is deliberately introduced in interferometry to make it easier to measure the wavefront error. Tilt An optical aberration caused by a simple tilt of the wavefront.There are several types of aberration measured by Quick Fringe: Any deviations from a perfectly flat wavefront indicates optical aberration.
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